The present disclosure relates generally to magnetic resonance imaging (MRI) and, more particularly, to a degenerate birdcage resonator for MRI.
A conventional MRI device establishes a homogenous magnetic field, for example, along an axis of a person""s body that is to undergo MRI. This homogeneous magnetic field conditions the interior of the person""s body for imaging by aligning the nuclear spins of nuclei (in atoms and molecules forming the body tissue) along the axis of the magnetic field. If the orientation of the nuclear spin is perturbed out of alignment with the magnetic field, the nuclei attempt to realign their nuclear spins with an axis of the magnetic field. Perturbation of the orientation of nuclear spins may be caused by application of radio frequency (RF) pulses. During the realignment process, the nuclei precess about the axis of the magnetic field and emit electromagnetic signals that may be detected by one or more coils placed on or about the person.
The frequency of the nuclear magnetic radiation (NMR) signal emitted by a given precessing nucleus depends on the strength of the magnetic field at the nucleus"" location. As is well known in the art, it is possible to distinguish radiation originating from different locations within the person""s body simply by applying a field gradient to the magnetic field across the person""s body. For the sake of convenience, direction of this field gradient may be referred to as the left-to-right direction. Radiation of a particular frequency may be assumed to originate at a given position within the field gradient, and hence at a given left-to-right position within the person""s body. The application of such a field gradient is also referred to as frequency encoding.
However, the simple application of a field gradient does not allow for two-dimensional resolution, since all nuclei at a given left-to-right position experience the same field strength, and hence emit radiation of the same frequency. Accordingly, the application of a frequency-encoding gradient, by itself, does not make it possible to discern radiation originating from the top versus radiation originating from the bottom of the person at a given left-to-right position. Resolution has been found to be possible in this second direction by application of gradients of varied strength in a perpendicular direction to thereby perturb the nuclei in varied amounts. The application of such additional gradients is also referred to as phase encoding.
Frequency-encoded data sensed by the coils during a phase encoding step is stored as a line of data in a data matrix known as the k-space matrix. Multiple phase encoding steps are performed in order to fill the multiple lines of the k-space matrix. An image may be generated from this matrix by performing a Fourier transformation of the matrix to convert this frequency information to spatial information representing the distribution of nuclear spins or density of nuclei of the image material.
MRI has proven to be a valuable clinical diagnostic tool for a wide range of organ systems and pathophysiologic processes. Both anatomic and functional information can be gleaned from the data, and new applications continue to develop as the technology and techniques for filling the k-space matrix improve. As technological advances have improved achievable spatial resolution, for example, increasingly finer anatomic details have been able to be imaged and evaluated using MRI. Often, however, there is a tradeoff between spatial resolution and imaging time, since higher resolution images require a longer acquisition time. This balance between spatial and temporal resolution is particularly important in cardiac MRI, for example, where fine details of coronary artery anatomy must be discerned on the surface of a rapidly beating heart.
Imaging time is largely a factor of the speed with which the MRI device can fill the k-space matrix. In conventional MRI, the k-space matrix is filled one line at a time. Although many improvements have been made in this general area, the speed with which the k-space matrix may be filled is limited.
To overcome these inherent limits, several techniques have been developed to simultaneously acquire multiple lines of data for each application of a magnetic field gradient. These techniques, which may collectively be characterized as xe2x80x9cparallel imaging techniquesxe2x80x9d, use spatial information from arrays of RF detector coils to substitute for the encoding which would otherwise have to be obtained in a sequential fashion using field gradients and RF pulses. The use of multiple effective detectors has been shown to multiply imaging speed, without increasing gradient switching rates or RF power deposition.
One such parallel imaging technique that has recently been developed and applied to in vivo imaging is referred to as SENSE (SENSitivity Encoding). The SENSE technique is based on the recognition of the fact that the spatial sensitivity profile of the receiving elements (e.g., resonators, coils, antennae) impresses on the spin resonance signal position information that can be used for the image reconstruction. The parallel use of a plurality of separate receiving elements, with each element having a different respective sensitivity profile, and combination of the respective spin resonance signals detected enables a reduction of the acquisition time required for an image (in comparison with conventional Fourier image reconstruction) by a factor which in the most favorable case equals the number of the receiving members used (see Pruessmann et al., Magnetic Resonance in Medicine Vol. 42, p.952-962, 1999).
A drawback of the SENSE technique, however, results when the component coil sensitivities are either insufficiently well characterized or insufficiently distinct from one another. These instabilities may manifest as localized artifacts in the reconstructed image, or may result in degraded signal-to-noise ratio (SNR). Accordingly, it is desirable to implement RF coil arrays and/or volume coils in MRI systems that (among other aspects) provide increased SNR with or without the use of parallel imaging techniques such as SENSE.
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by an apparatus for magnetic resonance imaging. In an exemplary embodiment, the apparatus includes a degenerate birdcage coil having a pair of opposing rings and a plurality of rungs positioned circumferentially around the pair of rings. Input excitation circuitry is used for applying excitation radio frequency (RF) energy to the degenerate birdcage coil at a first resonance mode of the coil. In addition, output receiving circuitry is used for receiving RF energy emitted by an object positioned within the degenerate birdcage coil. The output receiving circuitry receives the emitted RF energy at a plurality of resonance modes of the degenerate birdcage coil, including said first resonance mode. Thereby, the degenerate birdcage coil may be used as a phased array or for sensitivity encoding.
In another aspect, a degenerate birdcage resonator for magnetic resonance imaging includes a pair of opposing rings and a plurality of rungs positioned circumferentially around the pair of rings. A means for applying excitation radio frequency (RF) energy to the degenerate birdcage resonator is included such that a homogeneous RF field is established within the degenerate birdcage resonator. Furthermore, a means for independently reading each of a plurality of resonance modes of RF energy received by the degenerate birdcage resonator from an object placed therewithin is also included.
In still another aspect, a magnetic resonance imaging (MRI) system includes a computer, a magnet assembly for generating a polarizing magnetic field, a gradient coil assembly for applying gradient waveforms to the polarizing magnetic field along selected gradient axes, and a radio frequency (RF) transceiver system for applying RF energy to excite nuclear spins of an object to be imaged, and for thereafter detecting signals generated by excited nuclei of said object to be imaged. The RF transceiver system further includes a degenerate birdcage coil having a pair of opposing rings and a plurality of rungs positioned circumferentially around the pair of rings. Input excitation circuitry is used to apply excitation radio frequency (RF) energy to the degenerate birdcage coil at a first resonance mode thereof, while output receiving circuitry is used for receiving RF energy emitted by an object positioned within the degenerate birdcage coil. The output receiving circuitry receives the emitted RF energy at a plurality of resonance modes of the degenerate birdcage coil, including the first resonance mode. The RF energy received by the output receiving circuitry is then processed by the computer to produce MR images of the object to be imaged.
In still another aspect, method for implementing a degenerate birdcage resonator within a magnetic resonance imaging system includes applying excitation radio frequency (RF) energy to the degenerate birdcage resonator such that a homogeneous RF field is established within the degenerate birdcage resonator, and independently reading each of a plurality of resonance modes of RF energy received by the degenerate birdcage resonator from an object placed therewithin.